In a display device of various displays such as a liquid crystal display, a thin film transistor (TFT) that drives the display by applying a driving voltage is commonly used. In an activated layer therefor, a silicon-based material having excellent stability, processability or the like has been used.
However, since a silicon-based material absorbs light in the visible range, generation of carriers by incidence of light may cause a TFT to malfunction. If a light-shielding layer such as a metal is provided in order to prevent such malfunction, there are defects such as reduction in aperture ratio or an increase in power consumption since an increase in luminance of a backlight is required in order to keep the luminance of a screen.
For the production of a silicon-based material, even in the case of amorphous silicon that is believed to be capable of fabricating at low temperatures as compared with polycrystalline silicon, a high temperature of about 200° C. or more is required for film formation. Therefore, a polymer film having advantages that the cost is low, the weight is light and the film is flexible cannot be used as a substrate. Further, it has defects in respect of production that the device fabrication process at high temperatures involves a high energy cost and a long time is required for heating, or the like.
Under such circumstances, recently, in place of a silicon-based material, a TFT obtained by using a transparent oxide semiconductor has been developed. A transparent oxide semiconductor has attracted attention since it shows excellent properties as compared with conventional silicon-based materials. Specifically, it can be formed into a film at low temperatures without heating a substrate, can exhibit a high mobility of about 10 cm2/Vs, or the like. Among transparent oxide semiconductors, a field effect transistor obtained by using amorphous In—Ga—Zn—O (IGZO) that contains indium gallium, zinc and oxygen as constituent elements is regarded as a promising transistor since it can have a high on-off ratio. Further, as the material having a high mobility, a field-effect transistor obtained by using an In—Sn—Zn—O (ITZO)-based material has attracted attention.
In addition, an ITZO-based material is advantageous since the raw material cost is low as compared with an IGZO-based material. In order to reduce the raw material cost, as a sputtering target having a composition region in which the amount of In or Ga that is expensive is reduced, a sputtering target composed of a spinel structure compound represented by Zn2SnO4 and a hexagonal layered compound represented by In2O3(ZnO)m (m is an integer of 3 to 6) is known, as described in Patent Documents 1 to 3. However, these targets involve a problem that generation of nodules or abnormal discharge tends to occur easily.
On the other hand, in the sputtering target having the above-mentioned composition region, there is a tendency that the target strength, specifically, the transverse rapture strength, becomes low. The reason therefor is considered as follows. In a composition including an increased amount of Zn, a hexagonal layered compound having a relatively low strength is spattered, the scattered hexagonal layered compound triggers breakage. Such lowering in strength of a sputtering target is problematic since cracks may be generated by thermal strain by thermal history at the time of target bonding or by plasma radiation heat at the time of sputtering, stress applied by polishing or by other handlings. Under such circumstances, a sputtering target having a higher strength is desired.